Drill rigs indifferent sizes are frequently used in constructional work to drill holes in rocks. The drill rig comprises a rock drilling machine with a drill bit adapted for the specific conditions of the rock and used to penetrate the rock, and chip away the fractured rocks. The penetration force is generated by either a high level feed force or percussive wave forms. The rotation torque generated by a rotation motor will shear off the fractured rock.
In surface rock drilling processes, it is standard practice to use compressed air to blow away the rock cuttings and clean the surface of the rock to ensure that the drill bit always is in contact with a fresh and solid rock surface. If the cuttings were not cleaned away, the drill bit would penetrate the cuttings and further break down the cuttings into smaller sizes. This secondary breakage is not wanted because of the considerable quantity of waste energy and reduced drilling efficiency. The accumulation of the cuttings will furthermore resist the rotation of the drill bit and eventually jam the drill bit to stop.
A drill bit comprises flushing holes and compressed air is arranged to flow out through the holes during drilling. The flow of air is however stopped if the flushing holes get plugged by mud or if fine cuttings are compacted into the flushing holes. The ground condition where the rock drill works varies widely and the existence of mud in the ground substantially increases the risk of plugged holes. When the ground is soft while drilling rate is so fast that the air provided is not sufficient to clean the cuttings away, the drill bit is plugged by impacting the fine cuttings into the flushing hole.
When the flushing holes are plugged, the drill bit gets stuck and then the drilling is completely stopped. Then, the drill hole must be cleaned which takes a considerable time. It may involve pulling out the whole drill string. Consequently, it is important to ensure a continuous air flow through the flushing holes of the drill bit.
A commonly used method of detecting a reduced or stopped air flow in the flushing holes of a drill bit is to arrange an air flow monitoring device e.g. a venturi flow nozzle in the air flow path downstream a compressor. A low differential pressure is generated by such a device, in the range of a few psi while the working pressure is much higher. This normally means a differential pressure of less than 5 psi. A certain differential pressure indicates a normal air flow through the holes. When the certain differential pressure disappears, it indicates a stopped air flow. A mechanical or electrical device comprising a switch is used to detect a differential pressure and to send out a signal to a control unit to e.g. reverse the feed of air and stop the drilling. The creation and detection of a differential pressure puts demand on high sensitivity and high accuracy since the device used is working in a comparatively very high pressure environment.
Accordingly, prior art arrangements require a detection device having a very high sensitivity for detecting small pressure changes in a relatively high pressure environment. This kind of electrical and/or mechanical devices are very expensive and have high maintenance cost.
Furthermore, prior art monitoring devices give rise to a pressure drop in the air flow path which will have a negative impact on the air flow out through the flushing holes in the drill bit. It also has a negative impact on energy consumption.
Thus, there are needs to improve arrangements and methods for detecting a reduced, or stopped, air flow through the flushing holes of a drill bit in a rock drill rig.